The kidney is critical for maintaining calcium homeostasis. Most of the calcium ion (Ca2+) filtered at the glomerulus must be reabsorbed by tubules through both paracellular and transcellular pathways. The transcellular Ca2+ reabsorption occurring in the distal renal tubules accounts for ~10-20% of total reabsorption and is believed to be the primary target for regulation of calcium homeostasis by hormones (such as parathyroid hormone) and acid-base status. Transient receptor potential type V5 (TRPV5) channel localized to the apical membrane of distal renal tubules is a gatekeeper for transcellular Ca2+ reabsorption in the kidney. The overall long- term goal of our research is to understand the molecular mechanisms of regulation of TRPV5 in physiological and diseased states associated with disturbances of renal calcium transport. To this end, we will investigate the following 3 aims in the current proposal. Aim 1 will examine the mechanism and interrelationship between pH and Mg2+ regulation of TRPV5. Aim 2 will examine the mechanism of regulation of TRPV5 by parathyroid hormone (PTH). Aim 3 will examine the molecular mechanism of Klotho, an anti-aging hormone, in the regulation of TRPV5. We will use a combination of complementary biochemical, electrophysiological, and animal approaches. These studies are directly relevant to nephrolithiasis, since Mg2+ and alkali have been used in the treatment of kidney stone disease, and alterations in urinary Mg2+ and urinary pH influence urinary Ca2+. PTH is a principal calcitropic hormone but the mechanism by which PTH regulates renal Ca2+ reabsorption remains largely elusive. The study of Klotho will shed lights on our understanding of how Klotho can lower serum phosphorus (an important factor for longevity of life) without causing bone problems. Urinary Ca2+ concontration is critical determinant of kidney stone diseases. These studies will greatly advance understanding of TRPV5 biology and molecular regulation. Results of this proposal may provide improved undestanding of the process of stone formation and of treatment.